Photonic integrated circuits hold the potential of creating low cost, compact optical functions. The application fields in which they can be applied are very diverse: telecommunication and data communication applications, sensing, signal processing, etc. These optical circuits comprise different optical elements such as light sources, optical modulators, spatial switches, optical filters, photodetectors, etc., the optical elements being interconnected by optical waveguides.
Optical waveguides are typically implemented as solid dielectric light conductors, which allow to route light over the integrated optical circuit and to interconnect the various optical components integrated on the circuit. They also provide the interfacing between the optical fiber and the optical circuit, typically by physical abutment of the optical fiber to the waveguide. Due to the large difference in mode size between the optical fiber and the integrated optical waveguide, this typically leads to high coupling losses at the coupling interface.
Whereas these coupling losses can be kept within acceptable limits for low refractive index contrast optical waveguides, this is not the case for high refractive index contrast waveguide systems in which the fiber to waveguide coupling losses are in the order of 20 dB. However, these high refractive index contrast optical waveguide systems hold the promise of creating large scale integrated optical circuits providing lower cost and higher functionality compared to the low refractive index contrast integrated circuits.
Therefore, there is a great interest in improving the coupling efficiency between an optical fiber and the optical waveguide circuit. While various optical coupling schemes were originally developed for fiber-chip interfaces, these can also be applied to the optical coupling between an integrated optical waveguide and an integrated opto-electronic device (e.g. light source, modulator, optical amplifier, photodetector). Different technologies are presented in the literature to enhance the coupling efficiency to an optical fiber.
In a first approach, the optical mode of the single mode optical fiber is transformed to a smaller spot-size by using a lensed optical fiber or a high numerical aperture fiber. While these types of coupling interfaces provide lower coupling loss, the sub-micron alignment accuracy required to position the optical fiber with respect to the optical waveguide is very critical and implies therefore a high packaging cost of the integrated optical circuit.
Another approach is to use an integrated spot-size converter to expand the size of the integrated optical waveguide mode to match that of a single mode optical fiber. Both planar spot-size converter approaches and three dimensional spot-size converter approaches are applied. Three dimensional spot-size converters allow low coupling losses between the integrated optical waveguide and a single mode optical fiber, but fabrication of these components using standard planar waveguide technology is difficult. It has been shown that planar spot-size converter approaches allow low coupling losses to a standard single mode optical fiber in a low refractive index contrast material system. However, the use of this spot-size converter approach in high refractive index contrast devices always implies the need of a lensed optical fiber or high numerical aperture optical fiber, again resulting in the requirement of high alignment accuracy.
Moreover, these coupling approaches (physical abutment using standard optical fiber or lensed optical fiber and the use of a spot-size converter) all require a polished facet to couple light into the optical circuit. This excludes its use for wafer scale optical testing of the integrated optical functions to identify the known good dies on a processed wafer.
In order to improve the coupling efficiency to a standard single mode fiber in a high refractive index contrast system, and in order to relax the alignment accuracy of the optical fiber and to allow for wafer scale testing, one-dimensional grating structures have been proposed. These structures allow direct physical abutment from the top or bottom side of the structure with a standard single mode optical fiber, while the diffraction grating directs the light into the optical waveguide circuit. They allow coupling of light of a selected wavelength or wavelength band from a single mode optical fiber to a waveguide or from a waveguide to a single mode optical fiber.
The optical bandwidth of this type of devices is however limited by the dispersive nature of the grating structure, implying that the angle, under which light is coupled out of the grating, when excited from the optical waveguide, changes as a function of wavelength. Due to the limited numerical aperture of the optical fiber, wavelengths that deviate too much from the central wavelength, defined as the wavelength for which the angle of diffraction matches the tilt angle of the optical fiber, are less efficiently collected in the optical fiber.
Moreover, the performance of these one-dimensional gratings is critically dependent on the polarization of the light in the optical waveguide. Typically, only a single polarization state at a certain wavelength can be efficiently collected in the optical fiber, resulting in a very polarization dependent operation of the one-dimensional grating coupler. As in typical applications this polarization is unknown and varying over time, the applicability of the one-dimensional grating structures is limited. Only in the cases where polarization maintaining fiber is used or where a polarization scrambling approach is adopted, these one-dimensional gratings can be used. Also in the case where the one-dimensional grating structure is used to optically couple an integrated light source, generating, processing or detecting light with a known and fixed polarization, these devices can be used.
In order to circumvent the problem of polarization sensitivity, a two-dimensional grating coupler structure has been proposed (U.S. Pat. No. 7,065,272), which comprises two optical waveguides intersecting at a substantially right angle and a two-dimensional diffractive grating structure created at the intersection. When the diffractive grating is physically abutted with a single mode optical fiber, a polarization split is obtained that couples orthogonal modes from the single-mode optical fiber into identical modes in the first and second waveguide. While the ratio of coupled optical power between both optical waveguides is still dependent on the polarization of the incident light, this two-dimensional fiber coupling structure can be used in a polarization diversity approach, in order to achieve a polarization independent integrated circuit.
Besides the fact that fiber to chip coupling efficiencies comparable to the one-dimensional grating structure still have to be demonstrated, for some practical applications the optical bandwidth of the fiber-to-chip coupling efficiency is too small. This limited bandwidth is related to the intrinsic dispersive properties of the diffraction grating and the limited numerical aperture of the optical fiber. Whereas this is sufficient for applications requiring only a single optical wavelength or a set of closely spaced optical wavelengths, this is insufficient for other types of applications where the use of optical signals over a large wavelength span is required. This is typically the case for data communication applications where two different wavelengths, not closely spaced, are used for achieving a bidirectional data link. An important class of applications for which this is the case, is in Fiber-To-The-Home optical networks (FTTH), in which a 1310 nm upstream data signal, a 1490 nm downstream data signal and a 1550 nm downstream television signal are used, transmitted through one single mode fiber. Due to the large wavelength span used in this application the use of a prior art waveguide grating coupler structure is no longer applicable, as its optical bandwidth is too small.